What Makes a Skylight Energy Efficient? Traditional Skylights: Skylights use the same technologies as windows. But these technologies are even more valuable for skylights, which receive direct sun in summer and greater outside/inside temperature differentials in winter. Tubular Daylighting Devices: Increased Daylight. Tubular Daylighting Devices (TDD) gather sunlight at the roof and transmit it down to a diffusing lens mounted in an interior surface, usually a ceiling. The natural light from a TDD can illuminate closets, bathrooms, hallways, or other spaces that typically would not have direct access to sunlight, decreasing the need for electrical lighting.